This came years after the final Avengers book had been published in the
late 60s. Published by St. Martin's Press of New York, and written
by Avengers evergreen Dave Rogers with American collaborator John Peel,
this has recently been reprinted with a garish cover. Has a good
reputation among fans of such stuff.

From the back cover of the UK edition:
TARGET: JOHN STEED
There's a double agent afoot in England, and John Steed
thinks it's Mother. At least, somebody wants him to think so.
Mother thinks it's Steed. Dr. Keel thinks it's an ape, Emma Peel
and Cathy Gale think something very strange is going on, and Tara King
no longer knows what to think.
If you think The Avengers are back in action, you're
right on target.
In this first original Avengers novel in more than
a decade, all of the original crime-stoppers are back in action:
Cathy Gale's skills at judo are as sharp as always, as is Mrs. Peel's
wit, and Steed remains ever the dapper gentleman, from bowler to brolly.
But this is no Sunday outing. There are already
two dead agents, and nobody knows who can be trusted. It is going
to require the combined expertise of all the Avengers to repay the debts
incurred by these murders. Anything less than justice simply wouldn't
be sporting.
Just in time for the thirtieth anniversary of the celebrated
television show, Too Many Targets is a wonderful, timeless adventure,
rich in nostalgia, abundant in witty repartee and derring-do.
As Steed might say: "Mrs. Peel - you're needed
more than ever."